Beamforming is the process of using an array of antennas to control the direction of a transmitted signal. Signals transmitted from each of the antennas in the array constructively interfere to increase the combined signal strength in a desired direction while destructively interfering to decrease combined signal strength in undesired directions. Beamforming may be used in cellular communications systems to increase the capacity of users that may connect to a single base station. The base station may use beamforming to increase the capacity by simultaneously communicating on the same frequency band with multiple wireless terminals, such as cell phones, that are at different locations.
In order for a base station to perform beamforming, the channel state information (CSI) may be required. The CSI refers to the mathematical representation of a signal channel, namely the way in which a signal traverses a communication medium from a sender to a receiver. The CSI available at the base station for beamforming may be imperfect due to various problems, such as channel estimation error, quantization error, and delay in feedback. The imperfect CSI may increase the bit-error rate for a given signal-to-noise ratio compared with the bit-error rate that is achieved when perfect CSI is available at the base station. As such, it is desirable to perform beamforming using imperfect CSI with less increase in the bit-error rate for a given signal-to-noise ratio.